1. Field of the Invention
The invention relates to the field of remote data acquisition and, more particularly, to a system for periodically communicating data acquired by a remote data unit over a dial-up telephone line to a central computer and having demand reading capability.
2. Prior Art
There have been various attempts to provide remotely interrogable data acquisition units for the purposes of remotely and automatically reading data stored at these units. For example, in the utility industry, a remote data unit (RDU), also known as a meter interface unit (MIU), can be located at a customer's home or manufacturing site for accumulating utility consumption data (electricity, water, or gas) and for communicating this information back to a central computer located at the utility's home office. This type of system enables the utility to automatically and remotely read a customer's utility meter without having to send a person to the customer's location to physically read the meter.
Prior art automatic meter reading systems have generally used one of three forms of communication media. These are radio (RF), two-way interactive cable television (CATV), or dial-up telephone lines.
An example of an RF system is shown in U.S. Pat. No. 4,614,945, in which a mobile van interrogates an RF transponder attached to a meter to take the meter reading. However, this system suffers from the drawback that a vehicle must be driven within a few thousand feet of the meter in order to take its reading. Another drawback is that the RF transponder located on the meter is not suitable for use indoors due to attenuation of the low power 900 MHz signal used for communications. This limits the use of this device to meters which are located outdoors. In many areas of the country, however, meters (particularly water meters) are located indoors in basements.
The second type of system, which uses two-way interactive coaxial cable, is exemplified by U.S. Pat. Nos. 4,504,831 and 4,707,852. In these systems a remote data unit located at a customer's site periodically acquires utility meter consumption data and transmits it back to a central computer over a cable television coaxial cable. One drawback to this system is that it can only operate with a two-way interactive cable system since signal amplification is required from the customer's location back to the cable head-end. Also, two-way cable systems are relatively uncommon and therefore their use in automatic meter reading systems is limited.
Because of the foregoing limitations of RF and cable systems, the most effective remotely interrogable automatic meter reading systems in existence today utilize the dial-up telephone network as the communications medium. Telephone-based automatic meter reading (AMR) systems generally fall into two categories: so-called telephone "dial-outbound" systems and telephone "dial-inbound" systems.
A telephone dial-outbound system is exemplified by U.S. Pat. No. 4,582,152. In this system communication is established from a central (host) computer to a remote data unit located at the customer's site and connected to utility meters. Access to the RDU is made through a special subscriber test trunk which enables the host computer to be connected to a particular RDU without ringing a customer's telephone. This system has the advantage that the host computer can dial and access any RDU in the system at any time. This enables a utility to make a meter reading at-will, for example, when a customer has his service disconnected for the purposes of rendering a final bill. However, this system has the drawback of requiring special test trunk access circuitry which has to be installed at each telephone exchange within a utility's service area. This special circuitry enables the host computer to communicate with the RDUs connected through the particular exchange without putting a ringing signal on the customer's line. Another disadvantage is that this particular arrangement requires the cooperation of the local telephone company to purchase and install the special circuitry at each exchange.
An alternate to a telephone dial-outbound system is the telephone dial-inbound system. An example of such a system is shown in the above-mentioned U.S. Pat. Nos. 4,504,831 and 4,707,852. In one embodiment, a pseudorandom number generator is provided at the RDU which is used to randomly select a callback time for the RDU. When this callback time occurs, the RDU "wakes-up", dials the host computer's telephone number via a modem provided in the RDU and, upon making the connection with the host computer, transmits meter reading data back to the host. Randomization of the callback times is required since there can be several thousands of RDUs in a system. If all RDUs were to call back at the same time, only one would actually get through. By spacing the frequency of callback times sufficiently (e.g. once every thirty days) and using an appropriate pseudorandom number generation algorithm, the callback times can be spread out over the thirty day period, thus minimizing collisions in callback times. If a collision does occur, the RDU is programmed to pause for several seconds and retry the connection. If no connection is made after a predetermined number of retries, the RDU is reset.
One drawback to the aforementioned system is that throughput of the system is not optimized. Because of the nature of the randomizing sequence, it is possible that after an RDU is reset, it will call back in as few as two minutes or in as much as thirty days after the previous callback time. Without knowing the approximate time of reporting at the central computer, the database of meters which are expected to report in is difficult to manage. Furthermore, utilities generally try to schedule a reading of a customer's meter to fall on approximately the same day every month, so this system does not lend itself to accommodating a utility's usual meter reading practices. In addition, there is no way for the utility to call up the RDU and take an immediate (on demand) reading of a customer's meter.
Other telephone dial-inbound systems have attempted to improve the throughput of data transmission through various techniques. For example, in U.S. Pat. Nos. 4,241,237 and 4,455,453, an RDU is programmed to call the host computer at a precise real time, as indicated by an onboard real-time clock in the RDU. At the designated time, the RDU seizes the telephone line and transmits meter reading data to the host computer. The host computer then sends a signal containing the desired next real time of callback to the RDU where it is stored. The host computer also sends a synchronization signal to force the real time clock at the RDU to synchronize with a master clock associated with the host computer.
Similarly, U.S. Pat. No. 4,056,684 shows a telephone dial-inbound system for remote alarm monitoring. Alarm monitoring circuitry at a customer's home periodically dials into a host computer to indicate proper operation of the system. The host computer downloads a desired time interval to the remote alarm monitoring unit. The remote alarm monitoring unit, which has a clock, then waits the appropriate amount of time, as indicated by the time interval data sent from the host unit, until it calls back again.
One drawback to the foregoing systems is that some allowance must be made for drift in the onboard real time clock at the RDU. This drift can be up to 200 parts per million. Over a thirty day period, this implies a possible drift of several minutes plus or minus the nominal desired callback time. Because of this, the host computer has to set aside a several minute block of time for each RDU to call back to avoid collisions due to onboard clock drift. For example, with a two minute interval callback spacing, the maximum number of RDUs which can phone on a single line in a thirty day period without a chance of a collision occurring is 21,600. Such a system cannot accommodate many large utilities, some of whom have well over 100,000 customers.
One common problem faced by designers of telephone dial-inbound data acquisition systems is the requirement that the RDU not interfere with the normal operation of a customer's telephone. Not only can such interference be annoying to a customer, who may find he cannot use his telephone when the RDU is attempting to access the host computer, but it can be potentially life threatening if the customer needs to use the phone to dial the police or fire department in an emergency. Therefore, many telephone companies require any sort of auxiliary device which is connected in parallel with the customer's telephone not interfere in any way with the normal operation of the telephone. Preferably, the RDU must be fail-safe in this regard--the RDU must not under any circumstance accidentally seize the line while a customer is using his phone, and must immediately release the line if the customer picks up the telephone handset to make a call.
In order to meet these requirements, some sort of "off-hook" detection circuitry must be employed. Circuitry for detecting whether a customer's telephone is on-hook or off-hook is shown, for example, in U.S. Pat. Nos. 4,469,917, 4,578,534 and 4,847,892. However, these circuits merely detect whether the voltage or other electrical characteristic of the telephone line has dropped below a predetermined level which is assumed to indicate an "off-hook" condition. However, the line voltage is dependent upon the type of telephone system, the particular electrical characteristics of the customer's line, the loading on the telephone system, and the type of equipment connected, thus it can vary significantly. Because of these factors, these types of circuits may falsely indicate an "off-hook" condition when, in fact, no such condition exists, or fail to detect the customer attempting to gain access to the line.
Other types of dial-inbound telephone-based automatic meter reading systems utilize a so-called "polling" or "interrogate and callback" scheme. In U.S. Pat. No. 4,469,917 a remote data unit is placed in an "alert" condition for a short period of time each day during which it waits for a single ringing signal to be transmitted from the host computer to the RDU. The first ring is intercepted so as not to ring the customer's telephone. If no subsequent ring is heard within a predetermined time period, e.g. five seconds, the RDU then immediately dials back the central computer and transmits the data to the host computer. In U.S. Pat. No. 4,345,113 the first incoming ring on a customer's line is intercepted without ringing the phone. The RDU then answers the phone and waits to hear a special tone sent by a host computer. If this tone is heard within a predetermined period of time, the RDU sends its data back to the host computer. If the tone is not heard, the RDU assumes the call is a normal one and disconnects the RDU.
U.S. Pat. No. 4,578,534 describes a telephone dial-inbound data acquisition system in which an RDU detects and intercepts a single ringing signal and causes the RDU to call back the host computer if such a single ring is detected. If more than one ring is detected, the RDU shuts down and allows normal operation of the customer's telephone.
U.S. Pat. No. 4,847,892 describes a telephone based data acquisition system in which an RDU has a real time clock and calls a host computer on a periodic basis, e.g. once a month. In addition, once a day, the RDU is placed in a "standby" mode for a call from the host computer. In this "standby" mode, if a single ringing signal is detected the RDU immediately calls back the host computer to transmit its current data. In this fashion, scheduled periodic data transfers take place without any action of a utility or the requirement that the host computer call up the RDU to take the reading. However, the RDU is capable of being accessed on a daily basis during the predetermined "window" during which the RDU is in its "standby" mode. This enables the utility to take a daily reading of a utility meter.
Systems such as those described above, wherein the RDU intercepts, absorbs or in any way eliminates the first ring do, in fact, interfere with the customer's normal telephone service. Customers in effect pay for ringing service and each ring carries incremental value. For example, an elderly person may have difficulty getting to the telephone in time to receive a call if one ring is eliminated. Therefore, it is not likely that telephone companies would allow installation of such systems on a broad scale.
Systems such as those shown in U.S. pat. Nos. 4,469,917 and 4,578,534 which require the host computer to first call up the RDU, the RDU to be alerted, and the RDU to hang up and immediately call back the host computer are inefficient. This is because it can take upwards of thirty seconds to complete each dial-up circuit, i.e. placing the call from the host computer to the RDU and then placing the call from the RDU back to the host computer. This, combined with the necessary overhead for transmitting data from the RDU back to the host computer, means that a typical reading cycle can take up to ninety seconds. This substantially reduces the throughput (number of RDUs accessed per unit time) of the system. In addition, these systems all rely upon some type of single ring detecting circuitry which, in an effort to not annoy a customer, typically eliminates the first ring, so that only the second and subsequent rings will actually be heard by a customer if the ring was not intended for alerting the RDU. Such systems also have the disadvantage that the single ring alerting circuitry could fail in the off-hook mode after detecting the first ring, thereby rendering the customer's telephone useless.
The type of telephone dial-inbound meter reading system where the RDU automatically calls the host at a predetermined time could be the most efficient in terms of throughput, if it were not for the problems associated with clock drift. Although various attempts have been made to overcome this problem, none have achieved sufficient efficiency to enable a large number of utility meters, e.g. 100,000 or more, to be efficiently read during a one month period on a single incoming telephone line. In addition, with the heavy capital expense associated with installing a telephone-based automatic meter reading systems, utilities generally not only wish to be able to read their meters on a periodic basis (e.g. monthly) but also to take on-demand readings of a meter when necessary to address billing complaints or conduct special reads. This is particularly true in urban areas where there is a large movement of households where connect and disconnect calls are frequent, necessitating taking a meter reading on a particular day to prepare a final bill for a customer.
Since telephone-dial-inbound automatic meter reading RDUs must be capable of operation during periods in which telephone line power may fail or not be available, they normally require some sort of onboard battery for backup power. While the use of some types of batteries, such as lithium batteries, in combination with low power consumption integrated circuit devices, have extended the time period over which an RDU may operate without needing a battery change, it is a very desirable feature that a utility be alerted when the battery power for a particular RDU begins to fall below a critical threshold. In addition, utilities have indicated the desire to know if the RDU or its associated utility meter is being tampered with. In either of the foregoing cases, low battery voltage or tampering indication, it would be desirable that the RDU immediately dial the host computer and identify the problem and its nature.